The present invention relates to the production of oxygen or an oxygen-enriched gas, pressurized, from air or any other oxygen-lean gas, by using an electrochemical generator of the ionically conducting ceramic membrane type.
Such generators are well known today, and generally comprise one or more electrochemical cells for generating oxygen, each comprising one or more ceramic membranes. In such a generator, each ceramic membrane is heated, by external or built-in heating, to high temperature, and consists of a cathode (deposit or support), an anode (deposit or support) or a solid electrolyte (self-supported ceramic membrane or one supported on the anode or the cathode). Such a membrane (regardless of its shape) is supplied with air from the cathode side. The oxygen molecule dissociates at the cathode (deposit or porous support), the anion (O2−) diffuses through the solid electrolyte and recombines to form molecular oxygen at the anodic deposit or porous support. On the whole, by introducing atmospheric air at the inlet of such a generator, oxygen is generated that is pure, indeed of very high purity (grade N60, purity above 99.9999%), and optionally pressurized.
Such generators are described, for example, in documents WO 02/058830, WO 01/07149 and WO 02/058829 which can be referred to usefully as required. For example, the ceramic membrane is a solid electrolyte having the formula: (MαOβ)1-x(RyOδ)x 
where M is at least one trivalent or tetravalent atom selected from Bi, Ce, Zr, Ga, Th or Hf, α and β are such that the MαOβ structure is electrically neutral, R is at least one divalent or trivalent atom selected from Mg, Ca, Ba, Sr, Gd, Sc, Yb, Y, Sm or La, y and δ are such that the RyOδ structure is electrically neutral, and x is between 0.05 and 0.40, and more particularly between 0.06 and 0.20. Two families of solid electrolytes are conventionally employed: yrittria-stabilized zirconia (ZrO2-Y2O3, 5-15 wt % Y2O3), and gadolinium-doped ceria (CeO2—Gd2O3, so as Ce1.8Gd0.2O2-δ).
For example, each electrode, that is, anode and cathode, is a material with the formula: M1M2O3 
where M1 is one or more atoms selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Mg, Ca, Sr or Ba, and M2 is one or more atoms selected from Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn.
The family conventionally employed as an M1M2O3 type electrode-has a perovskite structure. Materials such as La1-xSrxMnO3-δ, La1-xSrxFeyCo1-yO3-δ, etc., can be mentioned as examples of anodic and cathodic materials.
However, the present invention, as described below, is not limited to this type of generator identified above.
The present invention relates more particularly to the production of ultrapure pressurized (1 to 50 bar) oxygen, particularly with a generator as defined above, providing flexibility of pressure and/or flow rate for the user.
Document U.S. Pat. No. 6,368,491 describes a ceramic membrane generator comprising n cells, mounted in parallel. According to the oxygen demand, one to n cells are used. A control system, or controller, is used to control or run the generator, by on/off control of each cell. No system is described for regulating the oxygen pressure.
Document U.S. Pat. No. 6,352,624 describes a ceramic membrane generator producing oxygen at a pressure of up to 138 bar, to which a storage tank is added, the combination being controlled by a control system, or controller, for determining the number of electrochemical cells necessary to reach the fixed oxygen flow rate. The control system does not allow control of the outlet pressure of the oxygen, which is supplied directly by the electrochemical cell or cells employed.
Document U.S. Pat. No. 5,855,762 describes a method for controlling a ceramic membrane generator, in which the electric current supplied to the said membranes depends on a signal indicating the quantity of oxygen produced by the generator and by the level of a tank for storing the oxygen produced. According to this solution, no means are used to control the outlet pressure.